The present invention relates to a device for applying adhesive material to sheets of packing material. In particular, the present invention relates to a device, hereinafter referred to as a "gumming device", designed for fitment on a packing machine, for applying adhesive material to predetermined portions of product packing material, e.g. blanks.
Known gumming devices, such as the one referred to in U.S. Pat. No. 2,276,997, feature a gumming roller having grooves for receiving adhesive material on predetermined portions of its periphery, and partially immersed in a vessel containing adhesive material in the form of liquid or paste. The gumming roller is usually provided with fixed scrapers for removing excess adhesive off the roller, the remainder of which, in the form of strips inside the grooves, is applied by the gumming roller on to predetermined surface portions of blanks of packing material.
Known devices of the aforementioned type present numerous drawbacks, mainly due to the presence of said fixed scrapers which, being subject to fairly rapid wear, soon result in the formation of a film of adhesive material on the surface of the gumming roller, and frequent machine stoppages for cleaning and maintenance. As described in the introduction to U.S. Pat. No. 4,249,547, one attempt to overcome the above drawback consists in replacing the fixed scrapers with a second scraper-transfer roller arranged tangent to and pressed firmly against the peripheral surface of the grooved roller, for at least partially removing the strips of adhesive material inside the grooves and at the same time prevent further adhesive from adhering to the peripheral surface portion of the second roller unaffected by the grooves on the grooved roller. The second roller then provides for transferring the adhesive glinging to it on to predetermined portions of blanks of packing material supplied successively by a third roller substantially tangent to the second.
Perfected gumming devices of the aforementioned type, however, have been found to present additional drawbacks, due, on the one hand, to the length and diameter of the two rollers involved, and on the other, to the relatively high contact pressure required between the two rollers for ensuring only the surface portion of the second roller operating in conjunction with the grooves on the gumming roller is coated with adhesive material.
As regards the length and diameter of the two rollers, it should be pointed out that the length must be at least equal to the length of the blanks measured along the axis of the feed roller, whereas the circumference of each roller must usually be at least equal to the width of the blanks. The width and length of the blanks, and consequently the diameter and length of the two rollers, may at times be relatively large. What is more, for a given contact pressure, the contact area between the two rollers increases alongside an increase in the diameter of the rollers. Consequently, for ensuring sufficient pressure is applied between the two rollers to prevent fouling the surface portion of the second roller unaffected by the grooves on the gumming roller, relatively high contact pressure must be applied, which, in most cases, results in distortion of the roller axes and, consequently, uneven distribution of the specific contact pressures applied.
In addition to requiring an extremely strong structure and fine surface finish, the two rollers on known gumming devices of the aforementioned type are therefore also subject to local wear, which drastically reduces the working life of the rollers, thus requiring frequent maintenance and replacement.